Ketones, unsaturated oxidation

Oxidation. Ketones are oxidized with powerful oxidizing agents such as chromic or nitric acid. During oxidation, carbon—carbon bond cleavage occurs to produce carboxyHc acids. Ketone oxidation with hydrogen peroxide, or prolonged exposure to air and heat, can produce peroxides. Concentrated solutions of ketone peroxides (>30%) may explode, but dilute solutions are useful in curing unsaturated polyester resin mixtures (see... 

Nitrile oxides are usually prepared via halogenation and dehydrohalogenation of aldoximes [11] or via dehydration of primary nitro alkanes (Scheme 1) [12]. However, it is important to note that nitrile oxides are relatively unstable and are prone to dimerization or polymerization, especially upon heating. 1,3-Dipolar cycioaddition of a nitrile oxide with a suitable olefin generates an isoxazoline ring which is a versatile synthetic intermediate in that it provides easy access to y-amino alcohols, )5-hydroxy ketones, -hydroxy nitriles, unsaturated oximes, and a host of other multifunctional molecules (Scheme 1) [5a]. Particularly for the formation of )5-hydroxy ketones, nitrile oxide-olefin cycioaddition serve as an alternative to the Aldol reaction. 

Noteworthy features of these reactions are the high predominance of E stereochemistry in the case of aldehydes and the good results obtained even with highly enolizable ketones (such as cyclopentanone), a, -epoxy ketones (isophorone oxide) and a,j8-unsaturated compounds (benzalacetophenone and cinnamaldehyde). 

The functions of flavoprotein enzymes are numerous and diversified.151-1533 A few of them are shown in Table 15-2 and are classified there as follows (A) oxidation of hemiacetals to lactones, (B) oxidation of alcohols to aldehydes or ketones, (C) oxidation of amines to imines, (D) oxidation of carbonyl compounds or carboxylic acids to a,(3-unsaturated compounds,... 

In the template reactions, acetone can be replaced by the preformed aldol, diacetone alcohol, and also the a, P-unsaturated ketone mesityl oxide.90-93 The use of hydroxyketones can be extended to include benzoins, provided that the l,3-diaminopropanenickel(II) complex is used (equation 13).91... 

Dimedone has a trivial name because its preparation is so easy that it was discovered early in the history of organic chemistry. The first step is a conjugate addition of diethyl malonate to the unsaturated ketone mesityl oxide (4-methylpent-3-en-2-one given a trivial name for the same reason). Ethoxide ion is the base for the usual reason that nucleophilic substitution at the ester group simply regenerates starting material. 

The overall sequence could be realized by 1,4-addition of a nucleophile to the enone and subsequent quenching at the a-position followed by B-elimination of the initial nucleophilic component. Such multistep processes will not be discussed here. However direct hydroxylation methods are scarce. Mo-riarty has reported that a,3-unsaturated ketones are oxidized by phenyliodosyldiacetate at the a-site in preference to the a -position, e.g. (138) to (139), although no yield has bWn indicated. There is no available mechanistic rationale, although the intermediacy of the a,B-epoxide is precluded. 

Isoxeaoks. Oximes of certain oi, -unsaturated ketones are oxidized by iodine-potassium iodide in aqueous THF containing sodium bicarbonate to isoxazoles. Thus treatment of the oxime of either (E)- or (Z)-P-ionone (1) with this combination of reagents gives the isoxazole (2) in 91 % yield. The expensive potassium iodide is needed only in catalytic amount. Reduction of (2) tvith sodium and 3 eq. of (-butanol in liquid... 

The required a-hydroxy ketones are accessible via a variety of synthetic methods, including the acyloin condensation (see Chapter 9), oxidation of ketone enolates, oxidation of enol ethers,and oxidation of a, 3-unsaturated ketones,as depicted below. 

In 2004, the N-heterocyclic carbene-catalysed internal redox reaction of epo Q aldehydes and a-haloaldehydes were developed by Bode et al. and Rovis et al., respectively. In 2012, Rovis group reported that NHC catalysed [4 + 2] hetero-Diels-Alder reactions of simple aliphatic aldehydes with a,p-unsaturated ketimines and ketones under oxidative conditions, giving the fra/2s-lactams and c/s-lactones in high yields with high enantioselectivities (Scheme 20.61). ... 

As described previously, acetone can be transferred to a,p-unsaturated ketone (mesityl oxide) by condensation and then hydrogenated to methyl isobutyl ketone over NiMgAl-LDO catalysts (536a). The same reaction can be accomplished in one step over Pd supported on MgAl-LDO (586,587). 

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